Am I over-sizing the MPPT... and should I?

Aphers

New Member
OK so my plan is to install two 320w panels, feeding a 4s LFP bank.
Crude maths (640/12=53) leads me to spec a 60A MPPT.
Real life numbers are different, though. I've never seen more than about 80% of rated output from a panel (admittedly, this is in not-so-sunny Scotland); further, my bank will not be charging as low as 12v (I hope).
Using say 85% output from the panels, and a charge voltage in excess of 13v, drops my current to 42A. So I would spec a 40A MPPT instead, and just accept the lost 2A. It's a pretty big cost saving, depending on what brand you're looking at.

But am I being penny wise and pound foolish. Will the bigger MPPT run cooler and have a longer, happier life?
 

snoobler

Solar Honey Badger
Moderator
40A would be fine. Many SCC readily tolerate being over-paneled. Important to confirm on whatever SCC you choose.
 

chrisski

Solar Addict
If you oversize your SCC with good planning on placement, you can go back and add panels that will be controlled by the same inverter Without purchasing a new one. Would just need to add something to combine the extra panels and a fuse if putting them in parallel.

I tried that with my RV roof panels, but when I measured for space, there was not enough space for the exact same panels, so I had to add an additional solar charge controller for the additional Panels. That is an example of not the best planning, but a work through.

Also, with the SCC, see if it can take extra voltage. My 100 volt, 50 amp Victron Charge controller can take 700 + watts for a 12 volt battery system or 1400 + watts for a 24 volt system.
 

RickP

Solar Power Padawan
I oversized my controller. It wasn’t terribly expensive to do so and gave me a level of easy expansion if needed. As you can see above, there are lots of options. This was a case where I felt extra was much better than not enough.
 

Aphers

New Member
Thanks all.
I will likely go with the smaller controller (actually given the prices involved I may go with two 20A).
My experience of trying to add panels to an existing setup has been that a couple of years down the line I could no longer get an exact matching panel (in my case, thanks to Brexit the German supplier I bought the first panel from would no longer deliver to me in the UK).
So I think that assuming that any future panels would need their own controllers is not a bad move.

Haven't chosen the SCC yet, but probably Renogy or EPever.
 

Supervstech

Administrator
Staff member
Moderator
If you choose two 20A controllers, get GOOD ones.
Many low price 20A controllers are garbage.
 

HRTKD

Boondocker
Thanks all.
I will likely go with the smaller controller (actually given the prices involved I may go with two 20A).
My experience of trying to add panels to an existing setup has been that a couple of years down the line I could no longer get an exact matching panel (in my case, thanks to Brexit the German supplier I bought the first panel from would no longer deliver to me in the UK).
So I think that assuming that any future panels would need their own controllers is not a bad move.

Haven't chosen the SCC yet, but probably Renogy or EPever.

I have three controllers in my RV trailer. Two of them are Victron 100/50 MPPT. If I changed panels (very unlikely), they have some room for more watts/volts. I would be more likely to add a new controller as any panels I added would never sync well with the existing panels.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
First off, I will suggest EPEver and please IGNORE Renogy, just do a search on problems with Renogy and that should answer that. EPEver is a good Value Grade line of gear that does exactly what it is supposed to and with little fuss.

Consider things ahead a bit. You may have one 12V Battery Pack and a 20A charger will charge it BUT is it enough ? Lot of folks trip on this point and start sounding like Drunken Sailors on a Saturday Night after having been at sea for 6 months. For example, a 12V/100AH LFP battery can take 50A Charge and will require 2 full hours to hit full from 0%SOC, which translates into 4 hours @ 25A charge rate, or 5 hours @ 20A. As you say "Sunny Scotland" is not known for long clear sunny days all the time and 5 hours of pure sunlight is fine in June but not happening in December.

I personally would suggest a more capable SCC that can do 50A or even 75A charge rate that can be dialed down via the SCC interface / software. Then as time moves along you can add more panels if needed for compensating for reduced Sunhours (we do this as a matter of course in Canada) and if you decide to add another battery pack you'll already be able to handle it with getting another SCC (plus breakers/fuses, witing etc) which tacks more onto the costs.

Hope it helps, Good Luck.
 

HRTKD

Boondocker
Agreeing with Steve on Renogy. A lot of their units seem wimpy and have no room for multiple solar panels or room for expansion.
 

Pyke13

Solar Enthusiast
I’m also in Scotland and have 530w and a 50a mppt. You’ve nae chance o it ever reaching max power even in summer. Most I’ve ever seen out of mine was around 30amps. That was down south in the middle of summer 2 years ago
 

Pyke13

Solar Enthusiast
I’m normally up north between Aberdeen and Inverness, the big worrie I’ve got is I still get a little power out of my solar at midnight in mid summer. I’m going to set my restart value at 12.5v for summer and 13.5v in winter. My mppt has a lithium and a user setting both can be programmed with charge setting but not with restart value. I will just change between the two about when the clocks change
 

Aphers

New Member
Thanks again for the replies. I wasn't aware of problems with Renogy, I recall Will giving them a decent review and saying they were a bit more user friendly than EPever.

Just for context, the plan is to take the boat out of Scotland for a few years of full time cruising. Certainly down to Spain and perhaps across the Atlantic. Also, the 640w is just what I am adding, there is already 230w on the boat, plus a towed hydro generator.

So the actual total is 870w, and I could add a third 320w panel without too much difficulty, but it would be under the boom and hard to keep unshaded.

Personally I would rather size the SCC correctly now, and not worry about expansion. If I don't expand, then I've wasted money. And if I do expand, running a separate SCC for the new panels gives me more options, e.g. different panel voltages, parallel and series strings, and of course if any of the equipment fails I haven't put all of my eggs in one basket.
 
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